Commercial passenger-carrying aircraft are typically powered by high bypass turbofan gas turbine engines for propelling the aircraft as subsonic speeds. A portion of inlet air is pressurized by a fan and bypasses the core engine for providing a majority of the thrust from the engine during operation. The remaining portion of the inlet air is channeled through the core engine Wherein it is compressed, mixed with fuel, and ignited for generating combustion gas from which energy is extracted for powering the core engine and the fan.
The fan size, or outer diameter thereof, is a primary factor in the maximum thrust capability of the engine. Larger fan diameter allows increased propulsion thrust from the engine, but also increases size and weight of the engine which adversely affect fuel burn by decreasing specific fuel consumption (SFC). Uninstalled SFC may be improved for subsonic turbofan engines as fan pressure ratio is reduced. However, as fan pressure ratio is reduced the airflow through the fan must increase to retain the required maximum thrust from the engine. This, in turn, requires a larger diameter fan which increases engine weight and increases nacelle scrubbing and interference drag. These effects diminish the uninstalled advantage of a low fan-pressure ratio engine to the point that when the engine is installed in the aircraft the overall efficiency of operation is diminishingly reduced.
Various types of fan arrangements are known and include, for example, fan blades having mid-span shrouds which radially divide the airflow into outer and inner portions, with the inner portion undergoing compression in a booster compressor for increasing the overall propulsion efficiency of the engine for obtaining improved SFC. However, the blade mid-span shrouds are typically integrally joined to the blade which requires a more complex and expensive manufacturing process. Furthermore, the additional centrifugal loads from the mid-span shroud must be carried through the blade dovetail and into the rotor disk which must be suitably sized for obtaining acceptable stress for a useful life.
More specifically, the blade includes an airfoil which is conventionally aerodynamically configured for obtaining a suitable pressure rise in the airflow channeled thereover during operation, with the airfoil ending at a root integrally formed with the dovetail which secures the blade to the rotor disk through a complementary retaining dovetail slot therein. The dovetail is a structural member configured solely for supporting the blade in operation and carrying the relatively large centrifugal loads from the blade into the rotor disk. The blade includes a transition in configuration between the airfoil and the dovetail, which transition is typically hidden by conventional flowpath platforms defining the inner boundary of the airflow over the blade airfoil. Accordingly, the aerodynamic efficiency of the blade is limited by the non-aerodynamic transition between the airfoil and dovetail required in each blade design for suitably carrying the centrifugal loads to the dovetail.